Scanning lithography system with opposing motion

ABSTRACT

A small field scanning photolithography system uses opposing motion of a reticle and a blank to compensate for image reversal by a projection system such as a conventional Wynne-Dyson optical system which forms a reverted image on a blank. The reticle has a reverted pattern. During scanning, the reticle moves along a reverted axis in a direction opposite the direction in which the blank moves. The opposing motions can either expose a stripe on the blank or index the reticle and blank for exposure of a next stripe. In one embodiment of the invention, the reticle and blank are on independently movable precision air bearing stages. Typically, the blank and reticle move together perpendicular the reverted axis and in opposite direction along the reverted axis. The stages can move the reticle and blank different amounts to correct for shrinkage and temperature changes which cause the size of the reticle and blank to differ.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure is related to and incorporates by reference in theirentirety co-filed and commonly owned U.S. patent applications Ser. No.08/412,238, unknown, entitled "Scanning Lithography System having DoublePass Wynne-Dyson Optics"; and Ser. No. 08/409,251, unknown, entitled"Magnification Correction for Small Field Scanning".

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to small field photolithography scanning and moreparticularly to systems and methods for scanning using an optical systemthat provides a reverted image.

2. Description of Related Art

The fabrication of flat panel displays (FPD's) for use e.g. in notebookcomputers is a well known process typically using photolithographytechniques similar to those used in integrated circuit (I.C.)processing. However, FPD's are typically fabricated on very largesubstrates (or blanks). A typical flat panel display measuresapproximately 8 inches×6 inches; and four or more such displays arefabricated on a single glass blank. Thus the blanks for flat paneldisplays are much larger than integrated circuits (ICs) and larger thansilicon substrates, which are typically no more than 12 inches indiameter, used in IC fabrication.

Conventional single projection lithography systems are ill suited forsuch large blanks because of the high cost of optics which can project aprecise image the size of the blank.

Conventional step-and-repeat lithography systems, such as thosemanufactured by Nikon Precision, Inc. and MRS, Inc., partition a blankinto blocks which are individually exposed. Step-and-repeat systemsallow use of optics with smaller field size (the size of a block) buthave difficulty aligning blocks precisely enough to provide uniformexposure at boundaries between the blocks. Accordingly, the speed of astep-and-repeat system may be reduced by alignments required after eachstep. Additionally, when patterns for different blocks differ, frequentreticle changes further reduce output. In a typical step-and-repeatsystem for IC manufacturing, each block is at least the size of acomplete IC or die to avoid boundary problems and reticle changes.

Optical systems with fields the size of a complete FPD are often tooexpensive to be practical. In contrast, small field scanning scans alarge reticle, equal in size to the blank, with unity magnificationoptics to expose overlapping stripes on the blank. Typically, smallfield scanning systems use less expensive optics with a field only aswide as a single stripe, typically about 30 mm.

Some small field scanning systems include Wynne-Dyson optics and imagereversing optic. Conventional Wynne-Dyson optics in such systems arebetween the reticle and the blank and include an input prism, an outputprism, a concave mirror, and optionally a lens. Light from the reticlereflects from the input prism to the concave mirror, back to the outputprism, and then out to form an image on the blank. A problem withWynne-Dyson optics is image reversal caused by the reflections. Imagereversal forms a reverted image having a left side which appears to bethe right side of the object, and vice versa. Conventional lithographysystems are unable to scan using a reverted image projected onto a blankand therefore must add optical elements to remove the image reversal ofthe Wynne-Dyson optics.

U.S. Pat. No. 5,285,236 entitled "Large-Area, High-Throughput,High-Resolution Projection Imaging System", to Jain, and U.S. Pat. No.4,171,870 entitled "Compact Image Projection Apparatus", to Burning etal., are incorporated by reference herein in their entirety and describeWynne-Dyson projection optics having output prisms (or roof prisms) withfaces that cause additional refection to remove the image reversal. U.S.Pat. No. 5,298,939 to Swanson et al., entitled "Method and Apparatus forTransfer of a Reticle Pattern onto a Substrate by Scanning", which isincorporated herein by reference in its entirety, describes using twoWynne-Dyson optical systems. A first Wynne-Dyson optical system forms areverted image of a reticle, and the second Wynne-Dyson optical systemreverses the reverted image to form a final image which is erect andnon-reversed.

The optical systems described in the above noted patents havedisadvantages. For example, roof prisms can cause multiple images if theprism is not precisely fabricated. A roof prism also increases the glasspath length which typically reduces field size. The optical systemdescribed in Swanson et al. doubles the cost for Wynne-Dyson optics anddoubles the space required between the reticle and blank.

Another concern in fabricating flat panel displays is that the blank istypically a glass such as a soda lime glass which unlike the siliconused in IC fabrication, is subject to significant compaction (shrinkage)during high temperature processing steps. Typical compaction of theseglass substrates during all of processing is e.g. approximately 10 to100 parts per million in length and width. The lithography techniqueswhich form patterns on a glass blank must properly align the variousmasks (reticles) to the blank which shrinks progressively duringprocessing. Typically, the masks or reticles are sized to the nominalsize of the image to be lithographed onto the blank. One approach makesa reticle size equal the expected blank size. This undesirably requiresaccurate characterization of the fabrication process to predict the sizeof the blank after shrinkage and is relatively expensive and complex.

An alternative is to accept misalignment due to the blank shrinkage.Accepting such misalignment is generally not considered desirable sincealignment errors result in degradation of the product and hence increasecost due to decreased yield.

In addition to the shrinkage of the glass blank caused by processing,another complication is that the temperature of a reticle when thereticle is made may differ from the temperature of the reticle duringthe projection onto a blank. The blank and the reticle expand andcontract with temperature, and a difference between the coefficients ofthermal expansion for the reticle and blank further complicatesalignment. Even if the fabrication process has been characterized andthe reticles correspondingly sized, in the absence of tight temperaturecontrols during processing, alignment errors still occur.

Thus broadly a goal of lithography is to improve alignment between areticle (or other pattern source) and a blank when the blank and/orreticle are subject to dimensional changes during processing.

In flat panel display processing technology, the commercially availableMRS stepper (a step-and-repeat system) includes a feature called "panelscale" for a type of magnification correction for flat panel processing.This apparatus includes a non-telecentric object plane which allows thereticle to be moved toward or away from the optics to slightly alter themagnification and focus of the image of a reticle.

In the wafer processing industry, another technique is described in"Variable Magnification in a 1:1 Projection Lithography System" by JamesJ. Greed et al., SPIE Volume 334 Optical Microlithography--Technologyfor the Mid-1980s, 1982. A Perkin-Elmer apparatus is intended to correctfor temperature effects on reticles and wafers in a conventionalintegrated circuit semiconductor fabrication scanning mask aligner. Insuch scanning mask aligners, each scanning stripe exposes a pattern atleast as wide an entire I.C. die on the wafer. Scanning the image of thereticle across the wafer exposes patterns for multiple dice.

This differs from small field scanning such as used in fabricating flatpanel displays. For flat panel display fabrication, each individualdisplay on the blank is exposed by several overlapping stripes. Thus,small field scanning involves stripe abutment and alignment concerns notpresent in a conventional stepper systems and requires opticalprojection systems different from a stepper or a scanning mask alignerfor ICs.

Photolithographic techniques for small field scanning of large blanks,such as used for fabrication of structures including flat paneldisplays, multi-chip modules, ICs, and printed circuit boards, needimprovement.

SUMMARY OF THE INVENTION

In accordance with the present invention, a small field scanningphotolithography system uses equal and opposite motions of a reticle anda blank to compensate for image reversal by a conventional Wynne-Dysonoptical system or other optical projection lens system which forms areverted image.

For small field scanning, scanning moves a projection field in ascanning direction to exposes a stripe on the blank. Between exposingone stripe and the next, indexing shifts the field in an indexingdirection which is perpendicular to the scanning direction. In oneembodiment of the invention, the scanning direction is along a revertedaxis, the axis which is reverted by the optical system. In anotherembodiment of the invention, the indexing direction is along thereverted axis. During motion along the reverted axis, scanning orindexing, the reticle and blank are moved in opposite directions alongto the reverted axis. The reticle and blank are moved in the samedirection along a non-reverted axis perpendicular to the reverted axis.

One embodiment of the invention, contains an optical projection systemwhich reverts an axis in an object plane, a reticle mounted on a firststage, and a blank is mounted on a second stage. The reticle has areverted pattern to compensate for reversal by the optical system. Thefirst and second stages are capable of independent motion along thereverted axis, and a control circuit controls movement of the first andsecond state so that the stages move in opposite directions along thereverted axis.

In another embodiment of the invention, stages mounted on a belt systemhold the reticle and blank. The positions of the stages on the beltsystem are such that when the belt system rotates, the stages move inopposite directions. The stages can move the reticle and blank in adirection along the axis through which the image is reverted e.g. forindexing or when the opposite motions of the reticle and blank expose astripe on the blank. The stages may also be capable of motion along thedirection of motion of the belt system so that the motion of the reticlediffers in magnitude or rate from the motion of the blank. Thedifference in motion can keep the reticle and blank aligned duringscanning by compensating for shrinkage in the blank and temporarytemperature changes which cause the size of the reticle and blank todiffer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an illuminator, a reticle, projectionoptics, and a blank in a photolithography system in accordance with anembodiment of this invention.

FIGS. 2A, 2B, 2C, and 2D show views of alternative embodiments of thesystem of FIG. 1.

FIG. 3 shows a perspective view of another photolithography system inaccordance with an embodiment of this invention.

FIGS. 4A and 4B show quadrants of a lens from the embodiment of FIG. 3and relations between the quadrants and images formed.

FIGS. 5A and 5B show a ray trace diagram of an embodiment of a doublepass Wynne-Dyson optical system in accordance with an embodiment of thisinvention.

FIG. 6 shows ray traces illustrating vignetting caused by prisms in anembodiment of this invention.

FIGS. 7A, 7B, and 7C show perspective views of portions of a small fieldphotolithography scanning system having double pass Wynne-Dyson opticsand magnification adjusting optics in accordance with this invention.

FIGS. 8A and 8B show a ray trace diagram of an embodiment of a doublepass Wynne-Dyson optical system in accordance with an embodiment of thisinvention.

FIG. 9 shows a folding prism which contains magnification adjustingoptics in accordance with this invention.

FIG. 10 shows a folding prism with a base block to reduce vignetting.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Opposing Motion Scanning

FIG. 1 is a perspective view of major optical components in a smallfield scanning photo-lithography system 100 which employs opposingmotion scanning in accordance with an embodiment of this invention. Insystem 100, a blank 160, typically a semiconductor wafer, a circuitboard, or glass substrate coated with light sensitive photoresist oremulsion, is mounted on a support (not shown) parallel and opposite to areticle 120 on its own support having an opaque pattern 122 to betransferred to blank 160. A conventional illuminator 110 such as a laseror a mercury arc lamp illuminates a portion of reticle 120 with light atwavelengths capable of changing (or "exposing") the photoresist on blank160. Illuminator 110 may be movably mounted or may contain optics forchanging the portion of reticle 120 illuminated. Illuminator 110typically contains a field stop (not shown) which restricts illuminationof reticle 120 to a region having a preselected size and shape.

A conventional Wynne-Dyson optical system 170 collects the light passingthrough reticle 120 and forms a portion of an image 162 on blank 160.Wynne-Dyson optical system 170 includes an input prism 131, a lens 140,a mirror 150, and an output prism 132. Typically, lens 140 is aconverging lens, and mirror 150 is a concave spherical mirror. The termWynne-Dyson optical system as used herein includes a catadioptric lenswith one reflecting surface and is not limited to an optical systemwhere lens 140 and concave mirror 150 have approximately the same centerof curvature. Light collected by input prism 131 passes through theupper half of lens 140 before being reflected by concave mirror 150.Light reflected from concave mirror 150 passes through the lower half oflens 140, and output prism 132 reflects that light on to blank 160forming a portion of image 162 on blank 160.

The imaging properties of Wynne-Dyson optical system 170 is determinedprimarily by lens 140 and concave mirror 150. Typically, lens 140 andconcave mirror 150 have separation and focal lengths selected to form aunity magnification image on blank 160 because at unity magnification,symmetry of the optical system causes the transverse aberrations,distortion, lateral color aberrations, and coma to be identically zero.Optical system 170 has a field which is smaller than the size of pattern122 on reticle 120 and image 162 on blank 160.

Scanning by moving reticle 120 and blank 160 relative to illuminator 110and optical system 170 exposes a stripe as wide as the aperture ofoptical system 170 and as long as the distance moved. If the stripe isnarrower than the width of blank 160, the remaining portions of blank160 are exposed by indexing (i.e. shifting perpendicular to the stripe)either optical system 170 and illuminator 110 or reticle 120 and blank160. After indexing a next stripe is exposed.

During scanning, motion perpendicular to an optical axis 175 of lens 140and optical mirror 150 cannot be performed in a conventional manner,such as moving illuminator 110 and optical system 170 relative toreticle 120 and blank 160, because the combination of reflections byinput prism 131, concave mirror 150, and output prism 132 in opticalsystem 170 causes image 162 to be reverted along a reverted axis whichis perpendicular to optical axis 175. For image 162 to be correctlyformed, a left edge 126 of reticle 120 must be imaged on a right edge164 of blank 160, and a right edge 124 of reticle 120 must be imaged ona left edge 166 of blank 160. Printing pattern 122 mirrored along axis175 and moving reticle 120 and blank 160 in opposite directionsperpendicular to axis 175 corrects for the image reversal. Movingreticle 120 and blank 160 in opposite directions has the advantages ofoffering near recoilless operation during acceleration of reticle 120and blank 160 and allows use of relatively simple Wynne-Dyson opticalsystem 170.

FIG. 2A shows a perspective view of a photolithography system whichemploys projection optics, such as optical system 170, which forms areverted image. The projection optics are mounted in a lens housing 270which contains an aperture 210 through which light from illuminator 110reaches the projection optics. Lens housing 270 is mounted on aprecision stage 275 which moves the projection optics and aperture 210along a Y axis. The Y axis is perpendicular to the reverted axis of theprojection optics. Typically, stage 275 is an air bearing stage moved bya linear motor operated by a control unit (not shown). Positionmeasurement devices 247, such as laser interferometers, measure theposition of lens housing 270 and transmit the measurements to thecontrol unit.

Reticle 120 is mounted on a secondary stage 220 which is mounted on astage 225. Blank 160 is mounted on a secondary stage 260 which ismounted on a stage 265. The control unit controls movement of stages220, 225, 260, and 265 according to measurements provided by measurementdevices 242 and 246 and a conventional alignment system (not shown) inlens housing 270. Secondary stage 220 is a precision stage capable ofmovement along the Y axis and rotation about a Z axis for conventionalalignment operations which align reticle 120 with blank 160. Secondarystage 260 is a precision stage capable of movement along the X axis forfine motion control and movement along the Z axis to move the surface ofblank 160 to the image plane of the projection optics. Stages 225 and265 are precision stages, such as air bearing stages, which move alongthe X axis, the axis reverted by the projection optics. Movement alongthe X axis can be for scanning or indexing. The control unit controlslinear motors which move reticle 120 (stage 220) in a direction oppositeto the movement of blank 160 (stage 260), but a slight difference inmovement may be imparted to correct magnification as described below.

FIGS. 2B and 2C show profile views of another mechanism for movingreticle 120 and blank 160 during scanning or indexing. Reticle 120 andblank 160 are mounted on stages 220 and 260 respectively which aremounted on a drive belt system 250. In one embodiment, belt system 250moves reticle 120 and blank 160 in opposite directions while exposing astripe on blank 160. FIG. 2B shows starting positions of reticle 120 andblank 160 for a stripe. In the starting position, optical system 170forms an image of left edge 126 of reticle 120 on right edge 164 ofblank 160. Reticle 120 and blank 160 move in opposite directions whileoptical system 170 forms a stripe of image 162, until reticle 120 andblank 160 reach the position shown in FIG. 2C where optical system 170forms an image of right edge 124 of reticle 120 on left edge 166 ofblank 160.

Belt system 250 keeps motions and displacements of reticle 120 and blank160 equal and opposite which reduces recoil and vibrations which maydisturb imaging. Stage 220 or 260 can change the relative motion anddisplacements of reticle 120 and blank 160 to compensate for shrinkageof blank 160 or correct the magnification of optical system 170.Magnification correction is disclosed in greater detail below.

Once reticle 120 and blank 160 reach the positions shown in FIG. 2C,indexing either reticle 120 and blank 160 or illuminator 110 and opticalsystem 170 prepares blank 160 for exposure of another stripe. Stages 220and 260 can move reticle 120 and blank 160 parallel to axis 175.Alternatively, optical system 170 and the illumination on reticle 120move in a direction parallel to axis 175 during indexing. The directionof motion of belt system 250 is reversed for formation of the nextstripe.

FIG. 2D shows a profile view of capstan or frictional drive mechanismfor moving reticle 120 and blank 160 during scanning or indexing. In thesystem of FIG. 2D, stages 220 and 260 are mounted on precision bearings(not shown) and are in frictional contact with a roller 280. Theprecision bearings maintain the spacing and orientation of stages 220and 260 while rotation of roller 280 moves stages 220 and 260 inopposite directions.

Several variations of the scanning method disclosed above are possible.For example, movement of belt system 250 or roller 280 can performindexing, while stages 220 and 260 move reticle 120 and blank 160 alongaxis 175 to form stripes. Scanning can be performed in reciprocatingdirections, or in typewriter fashion in one direction.

Double Pass Wynne-Dyson Optics

FIG. 3 shows a small field scanning photo-lithography system 300according to another embodiment of the invention. System 300 includesconventional illuminator 110 which illuminates a portion of reticle 120containing an opaque pattern 322. A double pass Wynne-Dyson opticalsystem 370 in accordance with this embodiment forms on blank 160 anon-reverted image pattern 362 of pattern 322 from the light transmittedthrough transparent portions of reticle 120. Prism 331 internallyreflects incident light from reticle 120 so that the light passes thoughlens 340 to a concave mirror 350.

FIG. 4A shows a front view of lens 340. Light from prism 331 passesthrough a first quadrant 410 of lens 340 and is transmitted to concavemirror 350 (FIG. 3). Concave mirror 350 reflects light from quadrant 410of lens 340 back through a second quadrant 420 of lens 340 into foldingoptics such as folding prism 332 in FIG. 3. The light is internallyreflected off a first face 332A of folding prism 332. Lens 340 andconcave mirror 350 are selected so that an image of pattern 322 forms ata middle plane of prism 332. A second face 332B of prism 332 reflectsincident light through a third quadrant 430 of lens 340 back to concavemirror 350. Concave mirror 350 reflects light from quadrant 430 backthrough a fourth quadrant 440 of lens 340 into an output prism 333 whichreflects the light onto blank 160.

Light passes through lens 340 four times before being focused on blank160, which is the equivalent of passing twice through a conventionalWynne-Dyson optical system. The first pass, the optical path from theobject plane on reticle 120 to the intermediate image, is completelysymmetric with the second pass, the optical path from the intermediateimage to final image 362 on blank 160; and the second pass undoes theimage reversal caused by conventional Wynne-Dyson optics. FIG. 4A showsthe changes in orientation 412, 422, 432, and 442 caused by thereflections at prism 331, concave mirror 350, prism 332, and concavemirror 350, respectively. The orientation of the pattern 322 is the sameas the orientation of the image 362. Accordingly, conventional processesfor moving reticle 320 and blank 360 in the same direction relative toilluminator 110 and optical system 370 may be employed.

A trapezoidal aperture 444 shown in FIG. 4B is typically used to provideeven exposure during scanning. As is well known in the art, sloped sides444A and 444B of aperture 444 provide less light nearer the edges of astripe so that the combined exposure of overlapping stripes does notoverexpose (or underexpose) the edges of the stripes.

The size of aperture 444 and therefore the width of stripes exposedduring scanning depends upon the usable area of lens 340. An area 446 oflens 340 near the diameters which divide the lens into quadrants 410,420, 430, and 440 is unusable because vignetting caused by input andoutput prisms 331 and 333 as disclosed below. Aperture 444 maximizes thewidth of each stripe when the scanning direction is perpendicular to theparallel sides of aperture 444. For this orientation, an optical axis375 of lens 340 and concave mirror 350 is at a 45° angle with thescanning direction.

A typical goal for optical system 370 for use in flat panel displayphotolithography is a resolution of at least 2 microns and a useablefield size or stripe width, as determined by the mean width of aperture444, of at least 30 mm. The resolution requirement translates into anumerical aperture (NA) greater than 0.17 at a wavelength λ of 436 nmusing the rule R=0.8λ/NA.

In order to analyze double pass Wynne-Dyson optical system 370 in aconventional optical design program such as GENII or OSLO available fromSinclair Optics, Inc. of Fairport, N.Y., light rays are unfolded at thefour reflections in prisms 331, 332, and 333. This results in ananalogous layout having two identical lenses 340 facing each other withprisms 331, 332, and 333 replaced by appropriate glass thicknesses.Distortion and lateral color aberrations are absent because of thesymmetry of system 370; and, since the intermediate image plain istelecentric, coma is absent. The design task becomes optimizing thecurvatures, thicknesses, and glass types to reduce the optical pathdifferences across the pupil for different field positions at thewavelengths used, and to minimize the field curvature. A challenge ofthe design is placing the image and object planes (in the actual foldedconfiguration) outside the radius of concave mirror 350 whilemaintaining adequate thickness of the small low index elements andcontrolling the aberrations at the edge of the field.

Table 1 lists optical parameters of one specific embodiment of opticalsystem 370, which is optimized at the g (436 nm) and h (405 nm) mercurylines from illuminator 110. I-line designs are possible although moredifficult due to the limited selection of non-solarizing opticalglasses.

                  TABLE 1                                                         ______________________________________                                        Surface   Radius (mm) Distance (mm)                                                                              Material                                   ______________________________________                                        501       ∞     15.173 (Object)                                                                            Air                                        502       ∞     120.1329     LAKN7                                      503       ∞     26.1855      KF6                                        504       -125.9625   143.5505     SF2                                        505       -280.7935   727.8927     Air                                        506       -1010.6241  -727.8927    Air                                        505       -280.7935   -143.5505    SF2                                        504       -125.9625   -26.1855     KF6                                        509       ∞     -101.7174    LAKN7                                      510       ∞     O (Intermediate image)                                  ______________________________________                                    

Each row of Table 1 identifies a surface shown in FIG. 5A, a radius ofcurvature of the surface, a distance to a next surface, and a materialtraversed to reach the next surface. In Table 1, negative thicknessesindicate rays traveling backward after the reflection from primarymirror 350 at a surface 506. SF2, KF6, and LAKN7 are respectively alight flint glass, a dense flint glass, and a dense crown glasscommercially available from Schott Glass Technologies, Duryea, Pa.

The rays shown FIG. 5A travel from an object plane 501 at reticle 120,through input prism 331 of LAKN7 glass, through compound lens 340 havingelements of KF6 and SF2 glasses. Compound lens 340 contains an achromatbetween mirror 350 and the center of curvature of mirror 350 to correctfor chromatic aberrations between the 436 nm and 405 nm wavelengthsused. Light rays from lens 340 reflect off surface 506 of mirror 350 andpass back through compound lens 340, into folding prism 332 of LAKN7glass where an intermediate image is formed at a middle plane 510 insidefolding prism 332. Output prism 333 which is also made of LAKN7 glass isomitted from FIG. 5A to better illustrate the light rays forming theintermediate image in folding prism 332.

FIG. 5B shows optical system 370 from middle plane 510 of folding prism332 out through output prism 333 to blank 160. The rays shown in FIG. 5Bare completely symmetric with the rays shown in FIG. 5A but traverse theelements in reverse order. The rays traced in FIGS. 5A and 5B are at afull 50 mm field height and set the scale for the lens drawing. Theprimary mirror is 269 mm in diameter. The length of the assembly, fromprimary mirror surface to surface of lens 340 to which the prisms 331,332, and 333 are bonded, is 898 mm.

Table 2 lists design and performance parameters for the embodiment ofTable 1.

                  TABLE 2                                                         ______________________________________                                        Field radius          50 mm                                                   Maximum useable field width                                                                         41.7 mm                                                 Numerical aperture    0.20                                                    Wavelength correction 436 & 405 nm                                            Maximum OPD           0.12 λ                                           Field flatness        2.6 microns                                             Telecentricity        3.8 mR                                                  Primary mirror clearance                                                                            1 mm                                                    Working distance      15.2 mm                                                 ______________________________________                                    

The field radius is the maximum image ray height from optical axis 375.The maximum useable field width is the longest chord that can be imagedafter the field size has been reduce to account for vignetting. Themaximum OPD (optical path difference) numbers include both wavelengthsat the same focal position and include contributions from fieldcurvature as well as all other aberrations. The field flatness is thetotal range, in both wavelengths, for the paraxial sagittal andtangential focuses across the unvignetted field. Some improvement can beexpected in both the OPD and field flatness performance if these valuesare optimized for only those field positions used in trapezoidalaperture 444 (FIG. 4B) for overlapping scans. The primary mirrorclearance is the available air space between the edge of primary mirror350 and reticle 120. There may be some opportunity to increase theprimary mirror clearance either by forcing the design, decreasing thenumerical aperture slightly, or by machining flats on the primary mirrorto provide a few more millimeters of clearance. The working distance isthe distance between the last optical surface (output prism 333) and theimage plane.

An unfolded description of optical system 370 is useful for analysisusing a lens design program such as GENII but does not account forvignetting caused by input, output, and folding prisms 331, 333, and332. However, vignetting can be calculated manually from the geometry ofprisms 331, 332, and 333 and optical information from a lens designprogram. Regions of the field for which principal rays are close to theprism boundaries, near the 45° angles or at the sides, are vignetted.

FIG. 6 illustrates a region subject to vignetting. A point 610 on anobject has a boundary ray 620 which is the limit of the cone of raysreflected by input prism 331 into optical system 370. Points closer tosurface 503 of input prism 331 are in a vignetted region having reducedintensity in the image formed by optical system 370. A width V of thevignetted region is given by the height of the axial marginal ray onsurface 503. This is just half the divergence of the ray bundle leavingan object point at a point where boundary ray 620 leaves prism 331.Aperture 444 of FIG. 4B blocks light in a vignetted region 446 of widthV. In the optical system described in Tables 1 and 2, input prism 331and output prism 333 set the limits for aperture 444 because the opticalpath length through half of folding prism 332 is less than that throughinput prism 331 and output prism 333.

Once width V of the vignetted region is known, the useable field iscalculated geometrically. FIG. 4B shows the available field afterL-shaped vignetted region 446 is removed. Trapezoidal aperture 444 hasbase length L which is a chord of circular boundary of lens 340. Themaximum available chord L in the available field region is given by##EQU1##

For the embodiment of Tables 1 and 2, chord length L is 41.7 mm. For atrapezoid height of 10 mm, the separation between scan lines, stripes,on a blank is 31.7 mm. There is a tradeoff between trapezoid height andscan separation, increasing the height of aperture 444 increase theamount of light through aperture 444 but decreases the separationbetween stripes. There is also a tradeoff between the numerical apertureand useable field size because vignetted field width V is proportionalto the divergence of the rays from the object, which are focused byoptical system 370. A system design could change configurations from ahigh resolution, smaller field size mode to a low resolution, largefield size mode. By changing the diameter of an aperture stop (notshown) on mirror 350 and the size of the field stop on illuminator 110simultaneously, resolution can be traded for field size. A largeraperture stop would be used with a small field stop and vice versa.

The proportion of useable field area appears quite small for the DoublePass Dyson configuration. Two points should be kept in mind: first,because the optical path is folded twice, only one quarter of thecircular area of lens 340 is available; and second, the requirement thatthe object and image planes are parallel and separated by more than thediameter of the primary mirror increases path length and the resultantdivergence of the ray bundles. This second effect is common to knownWynne-Dyson optical systems.

The advantages of the Double Pass Dyson approach over a roof prismdesign such as described in U.S. Pat. No. 4,171,870 to Burning et al. ora double Dyson configurations such as described in U.S. Pat. No.5,298,939 to Swanson et al. are clear. A roof prism can cause multipleimages near the center of the field if the roof angle is not preciselyfabricated. A roof prism also increases the glass path length in gettingthe image past the primary mirror which typically reduces field sizesignificantly. Inserting a field stop into the roof prism configurationwould be difficult. The double Dyson approach can provide a large fieldbut at the expense and complexity of building and aligning two lensesinstead of one.

Projection System with Magnification Correction Optics

FIG. 7A shows a double pass Wynne-Dyson optical system 770 in accordancewith another embodiment of this invention. Optical system 770incorporates magnification adjusting optics associated with an inputprism 731 and an output prism 733. The magnification adjusting opticsincludes two sets 710 and 720 of long radius lens elements. Each set 710or 720 includes a plano-convex element 712 or 722 and a plano-concaveelement 714 or 724 which define a narrow (between about 0.1 mm and 5 mm)meniscus air gap 713 or 723. In optical system 770, plano-convexelements 712 and 722 are bonded to prisms 731 and 732 respectively, andplano-concave elements 714 and 724 are movable relative to plano-convexelements 712 and 722 to adjust the magnification of optical system 770.In an alternative embodiment, plano-concave elements 714 and 724 arebonded to prisms 731 and 732 respectively, and plano-convex elements 712and 722 are movable.

The radii of elements 712, 714, 722, and 724 are equal, and sets 710 and720 introduce almost no power into optical system 770. The magnificationadjusting optics has a magnification neutral position where both airgaps 713 and 723 are equal and the optical system is perfectly symmetricabout the intermediate image plane in the center of folding prism 733.If both negative elements 714 and 724 are moved in the same directionone air gap 713 or 723 narrows, the other air gap 723 or 713 widens, andthe magnification and image field size of optical system 770 changesslightly.

A field stop 760 in a gap in the middle of folding prism 732 selects theportion of the intermediate image which forms a final image on blank160. FIG. 4B shows an exemplary trapezoidal aperture 444 for small fieldscanning. Field stop 760 eases the requirement of accuratelytransferring the image of a field stop in illuminator 110 to the objectplane (reticle 120). The alignment of field stop 760 in folding prism732 can be made with respect to lens 740 which is part of the sameassembly and makes alignment of input prism 731 and lens 740 toilluminator 110 less critical. Magnification adjusting optics 710 and720 and field stop 760 in the air gap in folding prism 732 areindependent, and alternative embodiments of this invention includeoptical systems incorporating one or the other.

In one embodiment, magnification adjusting elements 712, 714, 722, and724 are quarter sections of a conventional circular element and alignedwith input and output prisms 731 and 733 so that the optical axes ofelements 712, 714, 722, and 724, after reflection by prisms 731 and 733,are coincident with an optical axis 775 of lens 740 and mirror 750.FIGS. 7B and 7C show two perspective views of a portion of opticalsystem 770 illustrating the alignment of quarter section elements 712and 722. Aligning elements 712, 714, 722, and 724 with optical axis 775preserves more of the optical system's symmetry but may havedisadvantages in alignment and fabrication. For this alignment, thestationary point for magnification adjustment is along the optical axis.

Alternatively, elements 712, 714, 722, and 724 can be conventionalcircular elements aligned so that after reflections, rays along anoptical axis of elements 712, 714, 722, and 724 pass approximatelythrough the center of aperture 760. However, exactly aligning elements712, 714, 722, and 724 with the center of the trapezoidal aperture isunlikely, and some relative movement of the object and image withrespect to each element can be expected during magnificationadjustments. Accordingly, changing the magnification moves the imagerelative to the object, and reticle 110 and blank 160 must be alignedafter magnification adjustment.

Table 3 shows structural parameters of an embodiment of optical system770 of FIG. 7A when elements 714 and 724 are in a magnification neutralposition.

                  TABLE 3                                                         ______________________________________                                        Surface  Radius (mm)                                                                              Thickness (mm)  Glass                                     ______________________________________                                        801      ∞    10.3763 (Object)                                                                              Air                                       802      ∞    7.000           LAKN7                                     803      1999.9997  1.000           Air                                       804      1999.9997  6.000           LAKN7                                     805      ∞    112.0661        LAKN7                                     806      ∞    23.5396         KF6                                       807      -127.2866  143.5363        SF2                                       808      -281.3477  727.0806        Air                                       809      -1008.2385 -727.0806       Air                                       808      -281.3477  -143.5363       SF2                                       807      -127.2866  -23.5396        KF6                                       810      ∞    -100.1653       LAKN7                                     811      ∞    4.000           Air                                       812      ∞    O (Intermediate image)                                                                        Air                                       ______________________________________                                    

Table 4 indicates performance and structural parameters of theembodiment of Table 3 when the axes of elements 712, 714, 722, and 724are aligned with axis 775. Analysis of a system where the axes ofelements 712, 714, 722, and 724 is offset from axis 775 provides similarperformance.

                  TABLE 4                                                         ______________________________________                                        Field radius       50 mm                                                      Maximum useable field width                                                                      45 mm                                                      Numerical aperture 0.18                                                       Wavelength correction                                                                            436 & 405 nm                                               Magnification range                                                                              +/- 100 ppm                                                Maximum OPD        0.12 lambda, 0.08 lambda                                                      at 1X mag.                                                 Field flatness     2.4 microns                                                Telecentricity     2.3 mR                                                     Primary mirror clearance                                                                         15.6 mm                                                    Working distance   10.4 mm                                                    ______________________________________                                    

The parameters of Tables 3 and 4 are defined in the same way as theparameters of Tables 1 and 2 above. FIG. 8A shows a ray trace diagramfor optical system 770 from an object plane 801 to an intermediate imageplane 812 in the middle of folding prism 732, the plane of symmetry foroptical system 770. FIG. 8B shows a ray trace diagram from intermediateimage plane 812 to a final image plane 819.

The 1 mm air gap 713 between surface 803 and surface 804 is increased(decreased) while air space 723 is decreased (increased) in an equalamount to adjust the magnification of optical system 770 away fromunity. Changes of 0.158 mm in air gaps 713 and 723 cause a 100 ppmmagnification change. Since very little movement, less than 1 mm, isrequired for magnification adjustment, movable element 714 (and element724) can be mounted on flexure bearings and moved by a stepper motormicropositioner, such as those available from Melles Griot, Inc. ofIrvine, Calif. The amount of motion required for a given magnificationchange in this embodiment can be adjusted by varying the power of thefour magnification elements.

The optical system of Table 2 is similar to the optical system of Table4, but there are some notable differences. The numerical aperture in themagnification correcting lens has been reduced from 0.2 to 0.18. Thisreduces the vignetting, giving a larger useable field size of 45 mm. Thesmaller N.A. also reduces the diameter of mirror from 269 mm to 242 mmallowing for over 15 mm of clearance to image plane 819. The OPDs in theunity magnification condition are 0.08λ, giving additional aberrationbudget for magnification adjustments. For a 10-mm high trapezoid, thescan separation becomes 35 mm; for a 5 mm high trapezoid, scanseparation becomes 40 mm. The embodiment of Tables 3 and 4 was optimizedat the unity magnification condition only: an optimization over alladjusted magnifications would result in smaller maximum OPDs.

The introduction of an air gap between surfaces 811 and 813 insidefolding prism 732 can cause additional vignetting beyond what is presentfrom input and output prisms 731 and 732. This can be prevented bymaking the two halves of folding prism 732 large enough to accommodatethe field size and the divergence of the image cone through them. Theadditional glass length required by the optical design can be made up bycementing a block 732C of glass to the base of two prism halves 732A and732B as shown in FIG. 9. For embodiment of Table 3, the 100.1653 mm ofLAKN7 glass path length can be divided into 61 mm for the prism halve732A (or 732B) and 39.1653 mm for the base 732C. When this is done, thelimiting vignetting is caused by input and output prisms 731 and 733.

FIG. 10 shows an embodiment of magnification adjusting optics for adouble pass Wynne-Dyson optical system wherein a folding prism 1000contains two meniscus air gaps 1025 and 1035. Folding prism 1000contains a movable convex element 1030 between two plano-concaveelements 1020 and 1040 which are bonded to inside surfaces of portions1010 and 1050 of folding prism 1000, and moving symmetric element 1030adjusts magnification.

Movable element 1030 is equivalent to elements 712 and 722 (FIG. 7A)joined to form a symmetric convex element, and elements 1020 and 1040are equivalent to elements 714 and 724 (FIG. 7A). An alternative foldingprism with magnification adjusting optics has a movable symmetricconcave element equivalent to elements 714 and 724 (FIG. 7A) joined, andplano-convex elements equivalent to elements 712 and 722 (FIG. 7A)bonded to inside walls of the folding prism.

Placing magnification adjusting optics in folding prism 1000 hasdisadvantages. It requires a large cavity in folding prism 1000 toaccommodate three elements 1020, 1030, and 1040 which provides moreopportunity for vignetting. The intermediate image plane lies insidesymmetric element 1030 which must be thick enough to keep its surfaceand any dust particles on it out of focus. Further, the intermediateimage now moves inside element 1030, and there is no opportunity toinsert a field stop at that location.

The optical systems disclosed herein can be scaled in size by making theeach element larger to obtain larger field sizes. However, fabricationdifficulty and expense are important issues at sizes not too much largerthan about 1 meter diameter of the described embodiments. Increasing theN.A. of these systems is more difficult both from the vignetting andaberration standpoints. The optimum size for the trapezoidal height,which strongly affects the average width and thus the scan separation,must be addressed in conjunction with the illuminator design. Theextension of this approach to use i-line illumination should bestraightforward despite the more limited glass selection.

System with Relative Motion to Correct Magnification

For small field scanning lithography, relative motion of the reticle andthe blank may be used in addition to optical magnification adjustment tocorrect for compaction (or expansion) of a blank relative to thereticle. In particular, when the blank shrinks to a size smaller thanthe reticle, the magnification of the optical system should be decreaseto less than one, i.e. to the ratio of the reticle's size to the blank'ssize, and scanning should expose the reduced area of the blank, not anarea the size of the reticle. The relative motion of the reticle andblank can compensate for the reticle and blank having different sizes.For example, exposing a stripe on the blank is often performed by movingthe illuminator and the optical system along a scanning directionrelative to the reticle and the blank. Continuously moving the blank inthe scanning direction reduces the motion of the illuminator and opticalsystem relative to the blank. Relative motion of the reticle and blankis also advantageous to compensate for length differences along anindexing direction perpendicular to the exposed stripes.

Thus to provide full magnification correction in a small field scanningsystem, a combination of optical magnification correction and relativereticle-blank movement is advantageous. The relative motion between theblank and the reticle may be on the part of either. A slight relativevelocity is imparted to either the reticle or the blank as the two arebeing scanned, for instance by means of a secondary stage which movesthe reticle or blank relative to the other. The amount of relativemotion is proportional to the amount of magnification correctionrequired. Hence, any alignment error in the scan direction due to blankcompaction or expansion can be corrected.

The relative motion magnification correction is independent of theoptical magnification adjustment, and each may be used independently.Use of the relative motion magnification correction can selectivelycorrect for differences in one direction. For example, one could adjustthe separation of each scan stripe by a distance which divides the totalmagnification correction needed among all of the stripes in the indexingdirection. Without optical magnification correction, some residualuncorrected misalignment remains, i.e. the 10 to 100 parts per millionof the blank compaction across the diameter of the scan field, which istypically 30 to 40 mm.

Many systems for moving a reticle relative to a blank are possible, andthe embodiment disclosed herein of a secondary stage for the reticle orblank moving relative to the stage holding the other of the reticle orblank is merely one embodiment. Typically, relative motion is providedby a microstepper or other precision motor driving the secondary stageunder control of a conventional control mechanism receiving feedbackfrom the reticle alignment system.

The magnification correction schemes described here could be adapted tothe other optical configurations that have been discussed: roof prism,double Dyson, conventional Wynne-Dyson, and even telecentric 1Xrefractive optics.

Although the present invention has been described with reference toparticular embodiments, the description is only an example of theinvention's application and should not be taken as a limitation. Manyother embodiments of the invention are possible. For example, themagnification adjusting optics and/or magnification correction throughrelative motion of the blank and reticle in accordance with theinvention may be employed in variety of projection systems includingsystems with Wynne-Dyson optics and added elements to remove the imagereversal, Wynne-Dyson optics with opposing motion scanning, double passWynne-Dyson optics, and other projection optics. The magnificationadjusting optics can be attached to input and output prisms or can beinternal to the projection optics. Various other adaptations andcombinations of features of the embodiments disclosed will be apparentto those skilled in the art and are within the scope of the presentinvention as defined by the following claims.

We claim:
 1. A lithography system comprising:an optical systempositioned to form on a blank, a reverted image of an illuminatedportion of a reticle, wherein the reverted image is reversed along afirst axis and is not reversed along a second axis which isperpendicular to the first axis; and means for moving the blank and thereticle relative to the optical system to scan along multiple stripesthat collectively illuminate a Pattern on the reticle, wherein the meansfor moving comprises: means for simultaneously moving the reticle andthe blank in opposite directions along the first axis; and means forsimultaneously moving the reticle and the blank in the same directionalong the second axis.
 2. The lithography system of claim 1, wherein themeans for simultaneously moving the reticle and the blank along thefirst axis comprises:a first stage adapted to hold the reticle; a secondstage adapted to hold the blank; and a drive mechanism operativelyconnected to the first and second stages, to move the first and secondstages in opposite directions during scanning along the first axis. 3.The lithography system of claim 2, wherein the drive mechanismcomprises:a first linear motor connected to the first stage; a secondlinear motor connected to the second stage; and a control unit adaptedto control the first and second linear motor to move the first andsecond stages in opposite directions along the first axis.
 4. Thelithography system of claim 2, wherein the first stage further comprisesa first air bearing stage and a first sub-stage mounted on the first airbearing stage, and the second stage further comprises a second airbearing stage and a second sub-stage mounted on the second air bearingstage.
 5. The lithography system of claim 2, wherein the drive mechanismcomprises a belt on which the first and second stages are mounted. 6.The lithography system of claim 5, wherein the mechanism furthercomprises a stepper motor operably connected to rotate the belt, andthereby move the first stage in a first direction, and move the secondstage in a second direction opposite the first direction.
 7. Thelithography system of claim 6, wherein the first and second stages arerespectively capable of moving the reticle and the blank in a thirddirection perpendicular to the first and the second directions.
 8. Thelithography system of claim 7, wherein the optical system comprises aWynne-Dyson lens.
 9. The lithography system of claim 7, wherein theoptical system comprises:a lens having an optical axis substantiallyparallel with a surface of the reticle; a first reflector which reflectslight from the reticle through a first portion of the lens; a concavemirror which reflects light from the first portion of the lens backthrough a second portion of the lens; a second reflector which reflectsthe light from the second portion of the lens onto a portion of theblank where the reverted image forms.
 10. The lithography system ofclaim 2, wherein the drive mechanism comprises a roller coupled to thefirst and second stages so that rotation of the roller moves the firststage in a first direction and moves the second stage in a directionopposite to the first direction.
 11. The lithography system of claim 1,wherein the optical system comprises:a lens having an optical axissubstantially parallel with a surface of the reticle; a first reflectorwhich reflects light from the reticle through a first portion of thelens; a concave mirror which reflects light from the first portion ofthe lens back through a second portion of the lens; a second reflectorwhich reflects the light from the second portion of the lens onto aportion of the blank where the reverted image forms.
 12. The system ofclaim 1, wherein the optical system has an object field which isnarrower and shorter than a pattern of the reticle.
 13. The system ofclaim 1, wherein:the optical system has unity magnification; the meansfor simultaneously moving the reticle and the blank in oppositedirections moves the reticle and the blank equal distances along thefirst axis; and the means for simultaneously moving the reticle and theblank in the same direction moves the reticle and the blank equaldistances along the second axis.
 14. The system of claim 1, wherein:theoptical system has unity magnification; and the reticle has a size equalto a total of all areas of the bank to be exposed.
 15. A lithographymethod comprising:forming on a blank, a reverted image of a portion of areticle, wherein the reverted image is reversed along a first axis andnot reversed along a second axis; scanning the reticle and the blankalong a scan axis to expose a first stripe on the blank and form on theblank a first pattern that is a reverted image of a first portion of thereticle; indexing the reticle relative to the blank by moving thereticle and the blank perpendicular to the scan axis by a distance lessthan a width of the first stripe; and scanning the reticle and blankalong the scan axis to expose a second stripe on the blank and form onthe blank a second pattern that is a reverted image of a second portionof the reticle.
 16. The method of claim 15, wherein scanning comprisessimultaneously moving the reticle and the blank relative to an opticalsystem which forms the reverted image on the blank.
 17. The method ofclaim 16, wherein scanning along the scan axis further comprises movingthe reticle and the blank in opposite directions along the first axiswhile exposing a stripe.
 18. The method of claim 17, wherein the opticalsystem comprises a Wynne-Dyson lens.
 19. The method of claim 16, whereinscanning along the scan axis further comprises simultaneously moving thereticle and the blank together along the second axis while exposing astripe on the blank.
 20. A small field scanning systemcomprising:projection optics positioned to form on a blank a revertedimage of a portion of a reticle, wherein the reverted image is reversedalong a first axis and not reversed along a second axis, the projectionoptics having an object field that is narrower than a pattern on thereticle and an image field that is narrower than an area to be exposedon the blank; and means for scanning and indexing the object fieldacross the reticle to form on the blank overlapping stripes thatcollectively constitute a reverted image of the pattern on the reticle.21. The system of claim 20, wherein the means for scanning and indexingcomprises:means for simultaneously moving a reticle and a blank by equaldistances in opposite directions along the first axis to scan a stripe;and means for simultaneously moving the reticle and the blank by equaldistances in the same direction along the second axis to performindexing between exposures of stripes.
 22. The system of claim 20,wherein the means for scanning and indexing comprises:means forsimultaneously moving a reticle and a blank in opposite directions byequal distances along the first axis to perform indexing betweenexposures of stripes; and means for simultaneously moving the reticleand the blank in the same direction by equal distances along the secondaxis to scan a stripe.
 23. The system of claim 20, wherein the reticlehas a size equal to a total of all areas of the bank to be exposed.